Contract machine shops and OEMs rely on CNC milling machines to produce very precise components in industries such as medical, aerospace, automotive, motorcycle, and gun manufacturing. However, consistently achieving precisely machined components requires a quality retention knob, particularly in high-speed, high-volume applications.
In milling machines, retention knobs are commonly used with tapered tool holders to engage drawbars that hold the tool holder firmly in the spindle. The tapered tool holder comes with threads designed to accept the retention knob, but any improper seating of V-flange toolholders costs manufacturers in lost productivity. It also causes run-out and vibration that reduces the precision of machining while reducing tool & spindle life. This is an extremely common problem in machining.
Beyond this, high-speed machining in excess of 20,000 RPMs is often used in aerospace, medical, and other industries when machining exotic alloys and harder metals like titanium. At the higher RPMs, the precise and secure seating of tapered toolholders in the spindle (with the help of the retention knob) is even more critical. So much so, that failing to pay attention to this single detail can lead to further decreased productivity, less precise machining, reduced tool life, and even damaged workpieces.
That is why strong, well-manufactured retention knobs are so important. A superior product increases the rigidity of the set-up, which increases tool life and speeds cycle times (the measure of the time it takes to complete each part from start to finish.)
The problem is poorly designed retention knobs from overseas are often made of low-quality steel that may be prone to breaking during machining. Newer CNC milling machines exert significantly more drawbar pressure on retention knobs than in the past. As drawbar tension continues to increase in CNC machines, it puts so much pressure on the retention knob, that it may be more likely to break if not manufactured to the highest standard.
Retention knob failures may fail due to other reasons, including improper knob configuration, misaligned machines, metal fatigue, over-torquing, and exceeding the machine’s capability. That is why it is essential to periodically inspect each retention knob for signs of excessive wear, damage or corrosion. If found, the retention knob should be replaced.
“We deal with a lot of aerospace companies, some with high-speed CNC applications with harder metals or alloys,” says Bob Selleck, Manager at SAG Supply, a Long Island, NY-based provider of high-quality industrial tools, machinery, and accessories. “For that, you need a reliable retention knob that will last without breaking or moving. It needs to hold everything tightly in place with precision.”
Although retention knobs often look very similar, they are not interchangeable. The materials, coatings applied, and the quality of manufacturing can be important differentiators. Another consideration is where the item is manufactured, given that some overseas imports have a reputation for lower quality.
“Some [retention knob] imports do not always fit snugly or precisely in the flange and may wobble or move a little bit under high-speed applications,” adds Selleck.
The most reliable retention knobs are those made with superior U.S. steel and coated with black oxide, to prevent corrosion. In addition, the retention knobs should be subjected to the most rigorous quality control standards, including frequent inspection by the manufacturer. As an example, Selleck points to Ohio-based T.J. Davies, a manufacturer of high-quality retention knobs that uses certified 86L20 and 9310 steel drawn in the United States. He adds that SAG Supply carries TJ Davies retention knobs for their reliability at higher RPMs during CNC machining.
86L20, a low alloy nickel, chromium, molybdenum case hardening steel, has high hardenability without temper brittleness, along with good external and internal strength, and high wear-resistance. However, if a CNC operator desires and even higher quality steel, they can choose a retention knob made of 9310—a low alloy steel composed mostly of nickel and chromium. This allows also has high hardenability, core hardness, and fatigue strength, which makes it an excellent steel for use in heavy-duty machinery.
“We have found the domestic TJ Davies retention knobs to be of higher quality and more reliable than imports,” says Selleck. “They do not break; they last longer even at higher speeds with harder metals. We have carried them for over 10 years, and I have never heard of a problem or complaint.”
Long-term reliability also depends on using a superior coating to prevent corrosion. For this reason, each retention knob is coated with black oxide. In this process, the top layer of the ferrous material is treated with chemical that converts the surface to prevent rust. Black oxide also increases abrasion resistance while providing a decorative finish.
“I have never seen one of the black oxide coated retention knobs rust, and we sell a lot of them,” says Selleck.
If a retention knob ever needs to be replaced, it also helps to have quick shipping from a domestic supplier rather than dealing with lengthy overseas shipping and logistics. Manufacturers like T.J. Davies can ship items the same day with no minimum order and will drop-ship to end users on behalf of distributors.
“When I need retention knobs, I do not want to wait for overseas shipping,” says Selleck. “When I order domestically, I usually get what I need within two days.”
Because contract machine shops and OEMs require utmost CNC precision and productivity at all times, utilizing reliable retention knobs made with superior U.S. steel, coated with black oxide, can actually become a competitive manufacturing and production advantage.